Natural and Human Factors That Change Climate

Let's start in 1816

In June 1816, snow fell in Quebec City and parts of New England. Frosts killed crops through the summer, and food prices rose sharply in parts of Europe and North America. People called it "the Year Without a Summer".

The cause was thousands of kilometres away. In April 1815, Mount Tambora in Indonesia erupted, one of the largest volcanic eruptions in recorded history. It blasted sulfur gases high into the stratosphere, where they formed a haze of tiny droplets that reflected sunlight around the world.

The climate has always changed, for many natural reasons. To understand today's climate change, scientists must check each natural factor and ask: Could this explain what we're seeing now?

Natural factor 1: The Sun

The Sun's output (solar irradiance) is about 1361 W/m² at the top of the atmosphere, facing the Sun. It isn't perfectly constant:

• The sunspot cycle: about every 11 years, the Sun goes from few sunspots to many and back. Total output changes by only about 0.1% over a cycle, which causes a very small temperature wobble.
• Longer changes: from about 1645 to 1715, very few sunspots were observed (the Maunder Minimum). This quiet period overlapped with part of the "Little Ice Age", a cooler period in parts of the Northern Hemisphere, though volcanic eruptions also played a big role.

Since the 1950s, satellite and ground measurements show that the Sun's average output has stayed about flat or slightly decreased, while Earth has warmed rapidly. So the Sun can't explain recent warming.

Natural factor 2: Earth's orbit (Milankovitch cycles)

In the early 1900s, Serbian scientist Milutin Milanković calculated how slow changes in Earth's orbit change the amount of sunlight reaching different latitudes in different seasons:

• Eccentricity: the orbit changes from nearly circular to slightly oval, about every 100 000 years.
• Obliquity (tilt): the tilt of Earth's axis varies between about 22.1° and 24.5° over about 41 000 years. (It's about 23.4° now.) More tilt means hotter summers and colder winters.
• Precession: the axis wobbles like a spinning top, tracing a circle about every 26 000 years, which changes the time of year when Earth is closest to the Sun.

When northern summers are weak (less summer sunshine at high northern latitudes), winter snow doesn't fully melt, ice sheets grow, and their high albedo cools the planet further. Feedbacks, including changes in CO₂, amplify the small orbital change into a full ice age. About 20 000 years ago, during the last glacial maximum, almost all of Ontario was buried under the Laurentide Ice Sheet, more than a kilometre thick in many places. The Great Lakes basins were carved and shaped by these glaciers.

Milankovitch cycles act over tens of thousands of years, so they can't explain warming over a few decades. Based on orbital changes alone, Earth would be expected to be cooling extremely slowly right now.

Natural factor 3: Volcanoes

Large explosive eruptions can inject sulfur dioxide into the stratosphere, where it forms tiny sulfate aerosol droplets that reflect sunlight. Global temperatures can drop for one to three years, then recover as the aerosols settle out.

• Mount Pinatubo (Philippines, 1991) cooled the global average by roughly 0.5 °C for about a year or two.
• Tambora (1815) caused the Year Without a Summer.

Volcanoes also release CO₂, but far less than humans: all the world's volcanoes together release well under 1% as much CO₂ as human activities each year. Over very long geological times, though, volcanic CO₂ is a major part of the slow carbon cycle.

Natural factor 4: Plate tectonics, uplift and weathering

Over millions of years, the solid Earth reshapes the climate:

• Positions of continents: moving plates change where land and ocean are. When continents sit over the poles, ice sheets can form on them. Opening or closing ocean passages redirects currents. For example, when the ocean passages around Antarctica opened (tens of millions of years ago), a current began circling the continent, helping isolate it from warmer water, and Antarctica became ice-covered.
• Uplift and weathering: when plates collide, mountains are uplifted. Fresh rock exposed on mountainsides is broken down by chemical weathering. Rain (which contains a little dissolved CO₂, forming weak carbonic acid) reacts with silicate rock; the products wash to the ocean, where the carbon is eventually locked into limestone and other sediments. This slowly removes CO₂ from the air. Some scientists have hypothesized that the uplift of the Himalayas and Tibetan Plateau increased weathering and helped cool Earth over the last several tens of millions of years.
• Volcanic outgassing along plate boundaries adds CO₂ back. Over millions of years, the balance between outgassing and weathering acts like a slow thermostat.

These processes are far too slow to matter on human time scales.

Natural factor 5: Cosmic rays (a hypothesis that was tested)

Cosmic rays are high-energy particles from space. One hypothesis, proposed in the 1990s, is that cosmic rays help form tiny particles that seed low clouds, which reflect sunlight. When the Sun is more active, its magnetic field shields Earth from more cosmic rays, so there might be fewer clouds and more warming.

Scientists tested this idea. Laboratory experiments (including the CLOUD experiment at CERN in Switzerland) found that cosmic rays can help form particles, but the effect on clouds is too small to matter much for climate. And measured cosmic ray levels have shown no long-term trend that could explain recent warming. This is a good example of science at work: a hypothesis was proposed, tested and found not to explain the observations.

Human factors

• Burning fossil fuels (coal, oil, natural gas) for electricity, heating, transportation and industry releases CO₂ that was stored underground for millions of years. This is the largest cause of current warming.
• Deforestation releases carbon stored in trees and soil (especially when forests are burned) and removes a sink. It also changes the surface: replacing forest with fields changes albedo and the amount of water vapour released by plants.
• Forest fires: wildfires are natural in many ecosystems, but people cause many fires too (and use fire to clear land). Fires release CO₂ and soot. Soot that lands on snow and ice darkens it, lowering its albedo. A warmer climate brings longer fire seasons, so fires are both a cause and an effect of climate change.
• Industrial emissions include CO₂ (for example, from making cement and steel), methane leaks, nitrous oxide, and halocarbons. They also include sulfate aerosols from burning sulfur-containing coal and oil. Like volcanic aerosols, these reflect sunlight and cool the planet, hiding part of the warming from greenhouse gases. Cleaning up air pollution (which is good for health) removes some of this cooling.
• Agriculture releases methane and nitrous oxide (Lesson 8).